can we do it better?

Rolf BehlingRoyal Philips, Hamburg, Germany

New book: R. Behling. 2016. Modern Diagnostic X-Ray Sources: Technology, Manufacturing, Reliability. CRC Press, T&F, USA

History and Future of the X-Ray Tube:

Can We Do It Better?

AAPM Spring Clinical Meeting, SLC, 2016

1

Rolf Behling, Philips

Tubes – The Year After…Roentgen Frustrated

2

Roentgen‘s lab in Würzburg, GermanyDiscovery of the X-rays: Nov 8th 1895.

Curtesy of the German Röntgen Museum, Remscheid-Lennep, Germany

“Meanwhile,I have sworn so far, that

I do not want to deal with the behavior of the tubes, as these

dingus are even more capricious and unpredictable than the

women.”Prof. Dr. C.W. Röntgen, Jan 1897.

Translated from [7]


Questions WhichYou Have Never Asked

• How hot is a diagnostic X-ray beam?

• How much is a typical diagnostic X-ray photon?

• Isn‘t this cheap?

• How much does an LED photon cost, then?

• Will we produce X-rays in LED‘s ?

3

About 10 million Kelvin. That‘s why it is bio-destructible & difficult to produce.

No, extremely expensive

No, X-ray tubes will remain

If so, let‘s talk about better X-ray tubes

?

Two atto $ *)

*) 2 atto (10-18) $ (good CT tube), 6 atto $ (average CT tube)

A yocto cent **)

**) yocto = 10-24


• Computed Tomography (CT)– 70…150 kV, ~ 4 s scans, up to 120 kW, ~2 MJ– Gantry: centrifugal acceleration 30+ g– focal spot deflection

• Interventional– 60…125 kV– Minute-long pulse series, e.g. 20..80 kW, 5 ms @ 7,5 Hz– High tube current @ low tube voltage– Gyro forces

• General radiology– 40…150 kV, e.g. 80 kW, 3 ms every minute

• Mammo– 20…40 kV, small focal spots (0,1 … 0,3 mm)

~500 tube types on the market

The Modalities

4

Application Tube History X-ray Target Heat CathodeBearing CT Tubes Generator Failures Manufacture Recycle


Typical Rotating Anode Tube Assembly

5

-75 kV counter plug

+75 kV counter plug

Oil expansion bellow

X-ray portAluminum filter

Stator coils (motor)

Glass tube insert

CathodeW/Re -

compound anodeCopper short-circuit rotor

(motor)

Origin of X-rays (focal spot)

Leakage radiation protection (lead layer)

Aperture

e-

X-rays


X-rays

e-

Bearings in vacuum

X-ray tubehousing assembly

Vacuum

Oil

Oil


Who is Best in Class?

• GE– Thermo-ionic electrons (Coolidge, 1913)– Graphite anodes (CGR, later GE, 1967)– Largest anode (238 mm, 2005)

• Philips– Line focus (Goetze, 1919)– Metal frame + rotating anode (Bowers, 1929)– All metal ceramics (1980)– Spiral groove bearing (1989), dual suspended (2007)– Double quadrupole (2007)

• Siemens– Graphite backed anodes (1973)– Flat electron emitter (1998)– Rotating frame tube (2003)– Magnetic quadrupole, z-deflection (2003)

• Varian– Metal frames, largest anode heat capacity (1980ies)– Finned rotating anodes (1998)– Electron trap, anode end grounded tube (1998)

• Other vendors

6

Coolidge filament 1913

In alphabetical order

Curtesy: Siemens

Roentgen 1895

Bouwers rotating anode + metal1929

Goetze line focus1919

Metal ceramics 1980

Liquid bearing1989

Anode grounded 1998

Rotating frame 2003

120 kW,8° 2007



Bremsstrahlung, limited brilliance

Diagnostic Imaging

• Photons: energy > 16 keV• Bremsstrahlung (electron brake-radiation)

– Electrons accelerated in nuclear E-fields Continuous spectrum Re-fill of e- - shells adds characteristic lines e--scatter at free electrons (plasmons) heat Conversion efficiency 10-4

• Spatial resolution ∆x limited (dose penalty): – X-ray dose ∝(1/∆x)3…4

Source size ~ 0.2…2 mm

7

Nuclear Cloulomb potential in tungsten plane (schematic). Animation:1. Photon created upon hard inelastic

electron scatter2. Strong elastic scatter ( angular

diffusion)

Lamor formulaPlight=

𝑒𝑒2

6𝜋𝜋𝜀𝜀0𝑐𝑐3�̈�𝑿 2

Plight: Light intensity; �̈�𝑿 : Particle acceleration

e-

Clinic Tube Innovation X-ray Target Heat CathodeBearing CT Tubes Metric Failures Manufacture RecycleHistory

+22 megavolts

e-


Efficiency of Generation of Bremsstrahlung

8

• Good energy conversion by tungsten: atomic number 74, melts @ >3400°C

• Waste of electron energy through• inelastic low energy scatter, • Plasmon creation (electron cloud polarization, ~25 eV quanta)• Eventually energy transfer to phonons (heat)

Conversion efficiency ~10-4



Tungsten-Spectra

9

• Continuum of frequencies• Max photon energy = e− Vtube

• Tube voltage defines spectrum• Soft X-rays cancelled by filter

– Eliminating non-imaging photons– Key for patient safety– FDA: minimum 2,5 mm Al equiv.– Skin dose further down by additional up to 1

mm Cu– Strong filtration requires powerful tube

• e.g. interventional X-ray• TwinBeam CT)

Fine tuning high-voltage and filtration defines the bremsstrahlung spectrum

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

0 20 40 60 80 100 120 140energy [keV]

ph

oto

ns

per

(m

A s

mm

2 ) at

750

mm

80 kV100 kV120 kV140 kV

Spectrum vs. tube voltage. W-anode, 2 mm Al filter

W Lα,β


Soft radiation removed from the used beam by the X-ray filter


Minimize the anode angle αanode But: Observe heel effect, notably for CT

• Projected spot size counts– Projection on plane ⊥ central beam– X-ray fan usually narrow: 8° to 35°– High z-resolution @ anode shadow)

Long physical spot Lphysical = Lprojectedsin(αanode)

5 x…10 x gain of power and current Power ∝ physical spot length Low current density (reduced space charge)

Line Focus (Goetze)Resolution

10

αanode

Focal spot appears distorted, in the periphery of the field of view

e-

Cathode

Anode

Lphysical ≈ 5…10 x Lprojected


Projected FS defines sharpness


Why are reflection targets used for medical imaging instead of transparent targets?

20%

20%

20%

20%

20% 1. A.Reflection of electrons reduces target heating

2. The X-ray intensity is highest, given a small X-ray fan angle

3. There is backward enhancement of the X-ray intensity

4. No additional filter is needed, the target filters intrinsically

5. X-ray deflection enhances the brilliance

10

Rolf Behling, Philips 12


Bulk Anode Cooling

13

AnodeHeat storage

Electron impact

Heat radiation

Heat condution

Electron back scatter

100%

Glass tube, ball bearings

AnodeHeat storage

Electron impact

Heat radiation

Heat condution


30%

30%

40%

Single polar, liquid bearing

AnodeHeat storage

Electron impact

Heat radiation

Heat condution


60%

40%

Rotating frame tubeCourtesy: Siemens

Glass tube with ball bearings. Multiple exposures. Cooling: • Heat radiation starts strong at the beginning of the pause.• But, as the anode cools down (invisible, < 400 °C), heat

radiation ceases (T4).• Anode remains at elevated temperature.• The next patient faces a pre-heated tube limited performance • Solution: Heat conduction (dissipating residual heat)..


Radiation cooling: residual heat in the anode


Why is heat conduction cooling used in modern high performance X-ray tubes?

20%

20%

20%

20%

20% 1. It is simpler to manage

2. Heat radiation is not effective at high temperatures

3. The anode can be made smaller while maintaining high performance

4. Scattered electrons provide “heat conduction” 5. Improved image processing allows using stationary target

tubes in most cases

10


Historic “Mega Heat Units” are Out

• In 2010 IEC cancelled MAXIMUM ANODE HEATCONTENT (“Mega Heat Units”)

– Misleading metric – For historic technology

• IEC introduced new practical terms– Relevant in clinical use– Can be validated by the user

3.15 NOMINAL RADIOGRAPHIC ANODE INPUT POWER…POWER which can be applied for a single X-RAY TUBE LOAD with a LOADING TIME of

0,1 s and a CYCLE TIME of 1,0 min, for an indefinite number of cycles

3.16 NOMINAL CT ANODE INPUT POWER…POWER which can be applied for a single X-RAY TUBE LOAD with a LOADING TIME of

4 s and a CYCLE TIME of 10 min, for an indefinite number of cycles

3.20 CT SCAN POWER INDEX CTSPI

Please apply new IEC terminology

16

0

500

1000

1500

2000

2500

00 26 53 19 46 12 38 05

[ °C

]

[time / s ]

T(2)

T(3)

T(4)

V2

V3

V4

Focal spot

Focal track

Anode body

Typical CT scans: brain with an with out contrast

0

500

1000

1500

2000

2500

3000

00:00 00:07 00:14 00:21 00:28 00:36 00:43

[ °C

]

[ Zeit / time ]

T(2)

T(3)

T(4)

V2

V3

V4

Damage

0

500

1000

1500

2000

2500

00:00 00:07 00:14 00:21 00:28 00:36 00:43

[ °C

]

[ Zeit / time ]

T(2)

T(3)

T(4)

V2

V3

V4

Metal ceramics tube w/ liquid bearing

Same x-ray output + „MHU‘s“, but:• ball bearing glass tube (center) breaks: 1st scan ok, anode stays hot. •Tube with conduction cooling (SGB, bottom) survives. Cool at 2nd patient.

8 MHU

8 MHU

NEW



Anode Bearings in Vacuum

• Ball bearings– Hard steel, would freeze immediately w/o inter-layers Ag or Pb coating of balls– ~1 Watt heat conduction heat radiation cooling only– Limited life Start-stop needed– Deterioration by high speed, load, temperature

• Spiral groove bearing system (SGB)– Kilowatt heat conduction– ~10…50 µm gaps filled with liquid GaInSn

– Infinite rotation life, little wear at start & landing Continuous rotation (zero prep time)– Noiseless, stable, scaled to load & speed– Four bearings in one (2 x radial, 2 x axial), – Latest: dual suspended for CT (32 g)

(Rotating frame tubes have well lubricated ball bearings in oil)

Bearing type is key for tube lifeand practical experience (prep, cooling)

17

Ball bearing system in a glass tube BallsAxisRacewaySpring Cu cylinder

Two radial bearings of a liquid metal lubricated SGB.

Dual suspended SGB for high centrifugal forces in CT.

Gap with GaInSn water

Vacuum

Ball bearing unit, lead coated balls



Spiral groove bearings…

20%

20%

20%

20%

20% 1. A.are lubricated by cooling oil and thus enable heat dissipation by convection

2. B.allow for heat conduction even at low anode temperatures

3. C.cannot withstand high g-forces4. D.dissipate heat at stand-still and not when

rotating5. E.Need to stop between patients to conduct

heat out

10


Product Selection: Cheaper Might be Better

• Momentum of inertia Irotor of the anode rotor

Irotor ∝ Øanode4

Disadvantage for larger tubes with ball bearings– More prep time (pediatrics, interventional) – More heat for start / stop (air cooling)– Bearing failure– Costs

Select the right tube, not always the largest

20

Irotor : Momentum of inertiaØanode: Anode diameter

Better??



Larger anodes are always beneficial for tubes with ball bearings, right?

20%

20%

20%

20%

20%1. Yes. All major characteristics are superior, including prep

time and average power available for X-ray generation

2. No. The momentum of inertia is higher, and with it prep time and heat from start-stop operation

3. Yes, a large mass reduces bearing noise

4. Yes, a large momentum of inertia stabilizes the high rotor speed

5. Yes. More X-ray intensity can be produced while the overall heat dissipation stays the same.

10


Cathode

• Thermionic emission (e- boiled off W-emitter)

J = const * T2 * exp(-eф / kT)

• Child’s law: e- space charge in front of emitter

J = const * Vtube3/2 / dcathode-anode

2

• “isowatt point: tube voltage where space charge limit = anode limit (75 kV for the sample tube)

Cathode may be limiting tube performance

23

dcathode-anode: distance emitter – anode (e.g. 2 cm)Ifil: Emitter heating currentJ: Emitter current density (e.g. max 2 A/cm2)k: Boltzmann’s constantT: Emitter temperature (e.g. max 2500 °C)Ufil: Emitter heating voltageVtube: Tube voltage (< isowatt point space charge limit)ф: Work function of the emitting surface (e.g. 4,5 eV for W)

50kV

125kV

60kV

40kV

70kV80kV

90kV

100kV

0

50

100

150

200

250

300

350

4,2 4,7 5,2 5,7 6,2 6,7Ifil [A]

Itube

[mA]

0

1

2

3

4

5

6

7

8 Ufil [V]IFmax

Emission characteristics

of a 0,4 (IEC 60336) focal spot (11° anode angle, 108 mm anode Ø). Isowatt point 72 kV. Observe the Vtube

3/2 law in the space charge regime (right, hot emitter)

Anode limits(focal spot temperature)

Cathode limits(space charge)

Heating current ↔ Temperature

Potential

e-

e-

e-

e-

e-

e-e-

e-

Electron source Anode

Space charge deviation, reduced pull-field at the emitterdcathode-anode0

e-

e-

e-

e-

e-

e-

e-Heating current life time limit


0.1 s load

1900°C 2400°Ccathode anode


A low Isowatt Point, where cathode and anode limitations of the tube current meet, indicates …

20%

20%

20%

20%

20% 1. …good cathode performance. Space charge is minimal

2. …poor cathode performance. Space charge is maximal

3. …good anode performance

4. …a low conversion factor for electrical to X-ray energy

5. …having a poor X-ray high voltage generator

10


Focusing

• Electrostatic focusing (shape of cathode cup)– FS size independent of Utube (except w/ space charge)

• Recent: Magnetic focusing– magnetic quadrupoles– Magnetic deflection more projections less artifacts– Magnetic fields to be adapted to Utube

• MTF = modulation transfer function– Fourier transform of the projected intensity profile (point

spread function)– Measure of resolution capability (bandwidth of spatial

frequencies)

• Design goals– Focal spot independent of tube current (space charge)– Focal spot independent of tube voltage– Max. emitter size (tube life)– Minimal off-focal intensity

Electrostatic focusing is simpler, magnetic focusing is more effective and versatile

26

Electron beam tracing around one of the filaments. Geometry defines E-field for electrostatic focusing Dual filament cathode

Anode

Current density profile at the anode(focal spot exposure)



Anode grounded, drift space, electron trap

Off-Focal Radiation

• 2nd impact of back-scattered electrons• Artifacts @ hard contrast

– May mimic bleeding (stroke)– Shadows

• Remedy: electron trap, avoid mirroring

27

10% 90%

~10% off-focal (softer than primary)

Artifact: Over-compensated off-focal

Scattered electron trap Focal spot

Primary electron beam

Anode grounded tube. Scattered electrons are captured



Latest: Flat Emitter+Magnetic Focusing+Deflection

28

X-Rays

Scattered Electron Collector collects 40% of the primary electron energy

Simulated electron trajectories, Unprecedented compression, lowest isowatt point.

Cathode with tungsten flat emitter

Electrons

Z-deflection

Double quadrupole and dipole



A Rotating Frame CT Tube Assembly

29

“External” motor

Ceramics insulator (rotating)

Focal spot

Copper backed 120 mm anode (rotating, in contact with oil)

Radiation port

Rotating circular cathode(-70 kV)

Plastics insulator

Courtesy: Siemens

Rotor bearing (in oil)

Yoke of the magnetic quadrupole focusing and dipole deflection unit

e-

Type: Siemens Straton

X-rays


Rotating frame insert (+70 kV)Virtually “anode grounded”, as seen from the focal spot perspective

Rotating frame = Compact

Vacuum

Oil


A CT Tube with the Highest Power Density

30

Ceramics, magnetic field bridge (rotor inside)

Ceramics insulator-140 kV

200 mm segmented all metal anode

Titanium X-ray port

Cathode

Scattered electron trap

Central support plate

Double quadrupole magnet lens & dipoles

Dual suspended spiral groove bearing

Pinched-off tubing

Water in

electron drift path

Flat e- emitter

Focal spot on anode(inside electron trap)

Water out

e-

Type: Philips iMRC

X-rays


Latest: Top CT performance, reliable


Artifact Reduction by z/x FS Deflection

31 of 47z-DFS w/o z-DFS



Lack of Power, Gantry Speed Motion Artifacts

32 of 47



Lack of Power Image Noise

33 of 47



Why has the IEC canceled Maximum Anode Heat Content (slang: “Heat Units”) in 2010?

20%

20%

20%

20%

20% 1. IEC wanted to render the standard leaner

2. The terminology was accurate, but too complicated to communicate

3. The historic metric is misleading for current high-tier tubes.

4. It has not been used in practice anymore

5. The physics was faulty.

10


Tube Failures

36

Worn-out ball raceway and ball

0% 5% 10% 15% 20% 25%

bearing

arcing

other

manufacturing defect

frame / housing damage

filament

anode

vacuum leak

leaking cooling fluid

Anode crack (left), eroded focal spot track

Glass coatedarcing Arcing, craters

Broken filament

• Arcing• Low dose output• Beam hardening• Vibration / noise• Rotor frozen• Electron emitter fails• Implosion• Run-away arcing• Field emission >~50 µA• Heat exchanger error• Fluid leakage• Anode broken• Stator burn-out• Mechanical damage• other

Tube life time depending on concept, system type, usage, service, manufacturer

Broad failure distribution over time

Heat exchanger un-plugged compressed

Typical failure distribution of CT tubes, av. over tube types


Tube life time statistics of GE CT tubes in 13 CT systems in the Sloan Kettering Center, NYC


What can Cliniciansdo Better?• Select the right equipment

– State-of-the art metrics (no Heat Units anymore)• Minimize tube costs

– Single tube or multiple tubes systems?– Select long-life supplier (major differences)– Tube-included service contracts. Purchase photons, not iron

• Clinical application– Apply state-of-the-art de-noising technologies (power down)– Avoid cold-start @ high power– Avoid high tube current in angiography and mammography– Minimize fluoro time

37

How would you be driving your own car?


Röntgen 2016 …

38

Application Tube History X-ray Target Heat CathodeBearing CT Tubes Generator Failures (Manufacture) (Recycle)

”I am amazed,

quite a few 21st centurytubes are indeed excellent!


Thank You for Listening


can we do it better?

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